Laser scanning unit having automatic power control function

ABSTRACT

A laser scanning unit emits a laser beam for scanning a surface within a predetermined scanning range. The laser scanning unit has a laser diode, and a laser diode controller for controlling the laser diode to emit the laser beam. The laser beam has an intensity set in accordance with image data corresponding to one of a plurality of predetermined graduation steps within an intensity range defined by a first intensity and a second intensity greater than the first intensity. The laser scanning unit is further provided with a first adjusting system for adjusting the first intensity to become a first predetermined intensity, and a second adjusting system for adjusting the second intensity to become a second predetermined intensity. One of the first adjusting system and the second adjusting system is actuated during each scanning operation, and are actuated alternately during successive scanning operations.

BACKGROUND OF THE INVENTION

The present invention relates to a laser scanning unit which has anautomatic power control function for stabilizing the intensity of theemitted laser beam.

Laser beam printers are well known and widely used as printers employingan electrophotographic imaging method. The laser printer employs a laserscanning unit for emitting a modulated laser beam which scans a surfaceto be scanned. Specifically, the laser scanning unit has a laser diodewhich is controlled to emit modulated laser beam. The laser beam ismodulated on dot basis (on pixel basis) and the modulated laser beamscans a photoconductive surface in a predetermined direction. While thephotoconductive surface is scanned by the modulated laser beam, thephotoconductive surface is moved in the direction different from thescanning direction (e.g., in the direction perpendicular to the scanningdirection). As a result, a certain area is exposed to the modulatedlaser beam and a two-dimensional latent image is formed on thephotoconductive surface.

Recently highly qualified images have been demanded, and printers arerequired to form not only a black and white image, but a gradationimage.

In order to vary a density of the image for forming the gradation image,energy to be applied for each pixel to the photoconductive surface ischanged in accordance with the gradation level. Generally, there are twomethods for producing the gradation image with the laser beam printer.One of the methods, which is generally used, is a method of varying theduration of time during which the laser beam is emitted (the laser diodeis turned ON) without changing the intensity of the beam emitted by thelaser diode. Usually, a pulse signal is applied to the laser diode tocontrol the laser diode to emit the laser beam. The width of a pulsecarried by the pulse signal corresponds to the duration of time duringwhich the laser diode is turned ON. In this method, the pulse width ischanged in accordance with a gradation level.

The other method is to vary the intensity of the emitted laser beamaccording to the density of the each pixel of the image without changingthe width of the pulse. Recently, laser printers which are capableperforming printing operation at a high speed have been demanded. Forthe high speed printer, it is preferable to limit the pulse width to beas short as possible. Therefore, in the latter method, the pulse widthis fixed and the intensity of the laser beam emitted by the laser diodeis varied in accordance with the image to be printed.

FIG. 5 shows an example of an Intensity-Current characteristic (referredto as an I-C characteristics hereinafter) of a laser diode LD. The graphshows the intensity of the laser beam output by the laser diode, and anelectrical current available through the laser diode. In this example,it is assumed that when the intensity of the laser beam is smaller, thedensity of the produced image is lower, and that when the intensity ofthe laser beam is greater, the density of the produced image is higher.Further, in FIG. 5, an example of the input current (laser diode drivecurrent) and the corresponding output intensity is also indicated.

In the graph shown in FIG. 5, the intensity of the laser beamcorresponding to a white image is indicated as Pw, and the intensity ofthe laser beam corresponding to a black image is indicated as Pb. Therange of the intensity of the laser beam defined by the intensities Pwand Pb is divided into a predetermined number of gradations (e.g., 256),and according to the image data, the laser diode is controlled to emit alaser beam having one of above intensities for each pixel. Specifically,in order to control the laser diode to emit the laser beam having theintensity corresponding to the density of the image to be produced, thequantity of the electrical current available in the laser diode iscontrolled in accordance with the image data.

However, the I-C characteristic varies depending on the ambienttemperature. That is, if the temperature changes, the intensity of thelaser beam with respect to the electrical current available through thelaser diode varies, and therefore an entire image may not have a stable(constant) density. The I-C characteristic changes such that theinclinations of the characteristic lines do not change but they shift inright- or left-hand direction in FIG. 5, an example being shown bybroken lines. Further, the characteristic has a threshold current valueIth. If the electrical current available through the laser diode isgreater than the threshold current value Ith, the laser diode performsstably. That is, the performance of the laser diode is guaranteed, andthe intensity of the laser diode is substantially proportional to theelectrical current flowing through the laser diode. However, if theelectrical current available through the laser diode is less than thethreshold value Ith, the performance of the laser diode is unstable, andthe intensity of the laser beam may not correspond to the flowingcurrent. Furthermore, the threshold value Ith varies depending on thetemperature as mentioned above. AS the operating temperature increases,the threshold current increases from Ith to Ith' as shown in FIG. 5. Inorder to obtain a gradation image, the intensity of the laser beamshould be controlled strictly, and therefore above-described variationof the I-C characteristic should be taken into account when the laserdiode is driven. In other words, the intensity of the laser beam when awhite image is drawn should be maintained as Pw, and in order to obtainthe gradation image, the difference between the intensity Pb for a blackimage and the intensity Pw for the white image should be maintained tohave a predetermined difference.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved laserscanning unit which is capable of emitting a laser beam having a desiredintensity. Specifically, provided is the laser scanning unit with whichthe intensity of the laser beam output by the laser scanning unit withrespect to the electrical current available through the laser diode isstabilized regardless of the change of ambient temperature of the laserdiode.

For the object, according to the invention, there is provided a laserscanning unit for repeatedly executing a scanning operation where apredetermined scanning range of a surface to be scanned is scanned by alaser beam. The laser scanning unit includes a laser diode for emittingthe laser beam, a laser diode driver for driving the laser diode to emitthe laser beam which has an intensity corresponding to one of apredetermined graduation steps within an intensity range defined by afirst intensity and a second intensity greater than the first intensity,in accordance with an image data. Also included are a first adjustingsystem for adjusting the first intensity to a first predeterminedintensity, a second adjusting system for adjusting the second intensityto a second predetermined intensity, and an adjusting system controllerfor actuating one of the first adjusting system and the second adjustingsystem during each scanning operation.

Since two adjusting systems are provided, the upper and lower limits ofthe intensity range may be adjusted, and accordingly, accurate gradationimage can be produced always. Further, since only one of the first andthe second adjusting system is actuated during one scanning operation,even if the scanning speed is fast, an adjusting operation, i.e., anAutomatic Power Control (APC) operation can be achieved.

Optionally, the first adjusting system and the second adjusting systemare actuated alternately for successive scanning operations. Althoughonly one of the first and second adjusting systems is actuated at onescanning operation, since they are actuated alternately, both of theupper and the lower limits of the intensity can be adjusted.

Optionally, the first and second adjusting systems are actuated when thelaser beam does not scan the predetermined scanning area of the surfaceto be scanned.

Still optionally, the second adjusting system adjusts the secondpredetermined intensity to be greater than the first predeterminedintensity by a predetermined amount. In this case, the relationshipbetween the first and the second predetermined intensities aremaintained. Therefore, only by changing the first predeterminedintensity, and by examining the second intensity, both of the first andsecond intensities can be adjusted accurately.

Further optionally, if the image data represents a zero value, anintensity of the laser beam is less than the first predeterminedintensity. Specifically, if the image data represents a zero value, thelaser diode controller controls the laser diode not to emit the laserbeam. With this operation, if the surface to be scanned is aphotoconductive surface, the fog phenomena on the latent image may beavoided.

Still optionally, the first and second intensities are changeable.Specifically, the surface to be scanned is a photoconductive surface,and the first and second intensities are adjusted in accordance with asensitivity of the surface to be scanned. Alternatively, the change ofthe first and second intensities can be utilized for changing thedensity of the image to be produced.

Furthermore, the first intensity is greater than an intensity of a laserbeam which is emitted when a threshold current is passed through thelaser diode. Accordingly, the gradation image is formed with use of astably emitted laser beam, and the accuracy of the gradation levels maybe maintained.

Still further optionally, the first adjusting system may include adetector for detecting the intensity of the laser beam, a comparator forcomparing the intensity of the laser beam with the first predeterminedintensity, and a controller for controlling the driver to change theintensity of the laser beam in accordance with a comparison result ofthe comparator. Similarly, the second adjusting system may include adetector for detecting the intensity of the laser beam, a comparator forcomparing the intensity of the laser beam with the second predeterminedintensity, and a controller for controlling the driver to change theintensity of the laser beam in accordance with a comparison result ofthe comparator.

Furthermore, when the second adjusting system is actuated, the firstadjusting system is also actuated. Therefore, the change of thecharacteristic of the laser diode due to the temperature change can becancelled by adjusting the first intensity.

According to another aspect of the invention, there is provided a laserscanning unit for repeatedly executing a scanning operation where apredetermined scanning range of a surface to be scanned is scanned by alaser beam. The laser scanning unit includes a laser diode for emittingthe laser beam, a laser diode driver for driving the laser diode to emitthe laser beam which has an intensity corresponding to one of apredetermined graduation steps within an intensity range defined by afirst intensity and a second intensity greater than the first intensity,in accordance with an image data. The system further includes a firstadjusting system for being actuated to adjust the first intensity tobecome a first predetermined intensity, a second adjusting system foradjusting the second predetermined intensity to be greater than thefirst predetermined intensity by a predetermined amount, and anadjusting system controller for actuating a predetermined one of thefirst and the second adjusting systems during each scanning operation.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a schematic perspective view of elements included in a laserscanning unit as an embodiment of the present invention;

FIG. 2 is a timing chart showing an imaging duration and a duration forAPC within one horizontal scanning period (i.e., between two successivehorizontal synchronous signals);

FIG. 3 is a block diagram of the laser scanning unit;

FIG. 4 is the I-C characteristic and a modulation characteristic of alaser diode according to the embodiment;

FIG. 5 is the I-C characteristic and a modulation characteristic of alaser diode according to the prior art;

FIG. 6 is a graph showing the change of the I-C characteristic of thelaser diode;

FIG. 7 is a flowchart illustrating a first embodiment of the automaticintensity compensation procedure;

FIG. 8 is a flowchart illustrating a second embodiment of the automaticintensity compensation procedure; and

FIGS. 9A through 9F show a timing chart illustrating data communicationbetween a main CPU of the laser beam printer and a CPU of the laserscanning unit.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a schematic perspective view of elements included in a laserscanning unit SU as an embodiment of the present invention. According tothe embodiment, the laser scanning unit is employed in a laser beamprinter, and a photoconductive drum 4 of the laser beam printer is alsoshown in FIG. 1.

When an image data is transmitted from an image memory (not shown) tothe intensity modulating circuit 1, the intensity modulating circuit 1passes an electrical current through a laser diode 8 in accordance withthe image data. Specifically, the intensity modulating circuit 1 outputsa pulse signal, each pulse (i.e., the height of each pulse)corresponding to each pixel of the image, and amplitude modulated inaccordance with a gradation information contained in the image data.

A laser beam emitted by the laser diode 8 passes through a collimatorlens 2 which is arranged to receive the laser beam emitted by the laserdiode 1 and outputs a parallel beam. The laser beam output from thecollimator lens 2 is directed to a polygonal mirror 3. The polygonalmirror 3 rotates and deflects the laser beam in the directionperpendicular to the rotational axis of the polygonal mirror 3 as thelaser beam is reflected on the mirror surfaces formed on the outerperiphery of the polygonal mirror 3. The laser beam reflected by themirror surfaces of the polygonal mirror 3 passes through an fθ lens 5and scans a predetermined scanning area of the surface of thephotoconductive drum 4 in the direction indicated by arrow h (thescanning in this direction is referred to as a main scanninghereinafter). The photoconductive drum is rotatable about a rotationalaxis X. The main scanning direction is parallel to the axis X. While themain scanning is executed, the photoconductive drum is rotated. Thus,the surface of the photoconductive drum is exposed to the modulatedlaser beam and the latent image is formed. The latent image will bedeveloped to a toner image, and the toner image will be transferred ontoa recording medium.

In the laser scanning unit SU, a mirror 6 is provided for reflecting alaser beam which passed through the fθ lens 5 but directed to the out ofthe predetermined scanning area and does not contribute to forming ofthe latent image. The laser beam reflected by the mirror 6 is directedto a light receiving sensor 7 which outputs a predetermined signalindicative of reception of the laser beam reflected by the mirror 6.Based on the output signal of the light receiving sensor 7, a horizontalsynchronous signal HSYNC is generated.

FIG. 2 is a timing chart showing an imaging duration and a duration forAPC (Automatic Power Control) within one horizontal scanning period(i.e., between two successive horizontal synchronous signals). In theembodiment, within a period of time corresponding to one scan of thelaser beam, substantially a first half is used for forming the latentimage, and a second half is used for performing the automatic powercontrolling operation for adjusting the intensity of the laser beamemitted by the laser diode 8. In other words, one scanning operationincludes one image forming operation and one APC operation. Note thatthe APC operation is performed for every scanning operation. During theAPC operation, the current levels to be passed through the laser diodeis adjusted so that intensities of the laser beam have the lowestdensity and the highest density, respectively.

The former current level will be referred to as a white levelhereinafter. The white level is a level of current which does not causean attraction of a developing particle, such as the toner, onto theportion of the photoconductive surface which is exposed to the laserbeam corresponding to the white level. The latter current level will bereferred to as a black level hereinafter. The black level is a level ofthe electrical current which causes the toner to be attracted on thephotoconductive surface exposed to the laser beam, in a saturatedcondition.

FIG. 3 is a block diagram of a laser beam printer employing the laserscanning unit SU. The laser beam printer has a printer controller PC,and the laser scanning unit SU, which are connected by cables CB.

The printer controller PC has a CPU 30, an image clock 32, and an imagememory 31. The CPU 30 receives the image data transmitted from anexternal device such as a computer, develops the received image datainto a bit map image and stores the bit map image data in the imagememory 31. The image clock 32 outputs a data transfer clock CLK which isgenerated synchronously with the horizontal synchronous signal HSYNC.The bit map image data stored in the image memory 31 is read out by theCPU 30 synchronously with the transfer clock CLK on line basis, andtransferred to the laser scanning unit SU through a data line of thecables CB.

The laser beam printer is, as mentioned before, capable of producing the256-step gradation image. Each line of the image consists of a pluralityof pixels, and each pixel has information of 256-step gradation. Torepresent the 256-step gradation information, each pixel of the imagehas an eight-bit digital data D0 through D7. Accordingly, correspondingto each pixel of the image data, 8-bit image data is transmitted fromthe image memory 31 to the laser scanning unit SU. The image clock CLKis also transmitted from the image clock 32 to the laser scanning unitSU.

The polygonal mirror 3 is rotated by a motor 12. The motor 12 iscontrolled by the polygonal mirror control circuit 11 such that thepolygonal mirror 3 rotates at a predetermined rotation speed.

As described before, when the light receiving sensor 7 receives thelaser beam, the synchronous horizontal signal HSYNC which is referred tofor controlling writing start point of each line is generated by thewrite position detection circuit 13. The horizontal synchronous signalHSYNC is transmitted to the printer controller PU, and a CPU 150 of thelaser scanning unit SU.

The semiconductor laser 8 is packaged, unit containing the laser diodeLD and the photo diode PD, which are provided having a predeterminedpositional relationship. When the laser diode LD emits the laser beamtowards the polygonal mirror 3, another laser beam (subsidiary beam) isemitted in the opposite direction. The photo diode PD is arranged toreceive the subsidiary beam. Generally, the intensity of the subsidiarybeam is proportional to the intensity of the main beam (the beamdirected towards the polygonal mirror 3). Therefore, by detecting theintensity of the subsidiary beam, and adjusting the quantity of theintensity of the subsidiary beam, the intensity of the main beam can beadjusted.

In the embodiment, when the laser beam is incident to the photo diodePD, an electrical current corresponding to the intensity of the receivedlaser beam is generated in the photo diode PD. Due to the electricalcurrent flowing through a variable resistor 21, a voltage correspondingto the intensity of the laser beam received by the photo diode PD isgenerated across the variable resistor 21. The voltage generated acrossthe variable resistor 21 is applied to positive terminals of comparators14 and 15. To negative terminals of the comparators 14 and 15, outputvoltages of D/A (digital to analog) converters 16 and 17 are applied,respectively. Outputs of the comparators 14 and 15 are transmitted intothe CPU 150.

To the D/A converters 16 and 17, a predetermined digital data is input.The analog-converted values of the predetermined digital data areapplied to the comparator 14 and 15, respectively, as reference values.

Assume that the output voltages of the D/A converters 16 and 17 are afirst voltage V1 and a second voltage V2 where, V1 being less than V2.Then, based on the output of the comparators 14 and 15, it is determinedwhether the voltage across the variable resistor 21 is: lower than thefirst voltage; between the first and second voltages; or greater thanthe second voltage.

Specifically, the predetermined digital data input to the D/A converters16 and 17 are determined such that when the laser diode LD emits thelaser beam having the white level intensity, the voltage across thevariable resistor 21 coincides with the first voltage V1, and when thelaser diode LD emits the laser beam having the black level intensity,the voltage across the variable resistor 21 coincides with the secondvoltage V2. The white level intensity and the black level intensity mayvary depending on the sensitivity of the photoconductive drum 4.Accordingly, since the predetermined digital data is set to a defaultdata, the digital data is changeable. Further, by varying the first andsecond voltages V1 and V2, the density of the image finally formed onthe recording medium may be varied.

Utilizing the above, setting (adjusting) of the white and black levelsare executed, which will be described in detail later. The reason theresistor is connected to the photo diode PD is for canceling theintrinsic difference of the photo diode PD, and for adjusting therelationship between the voltage values of V1, V2 and the voltage acrossthe resistor 21. That is, even if the intensity of the laser beamemitted by the laser diode LD is the same, due to the intrinsicdifference of the characteristic of the photo diode PD, the voltageacross the variable resistor 21 may vary if the resistance of theresistor 21 is fixed. In order to cancel for such errors, or intrinsicdifference, by varying the resistance of the variable resistor 21, thevoltage across the resistor 21 is adjusted such that it is always thesame if the intensity of the laser beam is the same.

The laser diode LD is connected serially to a current source 19 througha switching element 18. When the switching element 18 is closed, thelaser diode LD emits the laser diode at an intensity which correspondsto the electrical current supplied by the current source 19. The currentsource 19 is a voltage-controlled current source. An output voltage ofthe adder 42 is input to the current source 19, and the current source19 supplies the electrical current which corresponds to the appliedvoltage. To the adder 42, an output voltage Vw of the D/A converter 41and an output voltage Vd of the D/A converter 44.

The output voltage Vw of the D/A converter 41 is set such that, if onlythe output voltage Vw is applied to the current source 19, theelectrical current flow through the laser diode LD will be greater thanthe threshold value Ith, and an image produced with use of the outputlaser beam will be recognized as a white image. Detailed description ofthis process will be provided later herein. If only the output of theD/A converter 41 is applied to the current source 19, the electricalcurrent available through the laser diode LD will be Iw as shown in FIG.4, and the intensity of the laser beam emitted by the laser diode LD isPw. FIG. 4 shows an example of an I-C characteristic of a laser diodeLD. As described above, the graph shows the intensity of the laser beamoutput by the laser diode corresponding to an electrical currentavailable through the laser diode. In FIG. 4, further to the above, theinput current (laser diode drive current) and the corresponding outputintensity according to the shown I-C characteristic is also indicated

The output voltage Vb of the D/A converter 43 is set such that, if sumof the output voltage Vw of the D/A converter 41 and the output voltageVb of the D/A converter 44 is applied to the current source 19, theelectrical current flow through the laser diode LD will be Ib (shown inFIG. 4). The image to be produced when the laser diode emits the laserbeam having the intensity of Ib will be recognized as a black image.Detailed description of this process will be provided later herein.

To the intensity modulating D/A converter 44, the output voltage Vb ofthe D/A converter 43, and the image data D0 through D7 are inputthereto. The intensity modulating D/A converter 44 is a multiplicationtype converter which divides the input voltage input by the D/Aconverter 43 based on the input image data D0 through D7. Specifically,the output voltage Vd of the D/A converter 44 is expressed as:

    Vd=DD×Vb/255

where, Vb is the output voltage of the D/A converter 43, DD is thedecimal value of the image data D0 through D7 (0≦DD≦255).

The output voltage Vadd of the adder 42 is expressed as follows.##EQU1##

When DD=0 (when each of the image data D0 through D7 is 0, i.e., a whiteimage), Vadd=Vw. When voltage Vw is applied to the current source 19,electrical current Iw is supplied to the laser diode LD and theintensity of the laser beam is Pw. When DD=255 (when each of the imagedata D0 through D7 is 1, i.e., a black image), Vadd=Vw+Vb. When voltageVadd (=Vw+Vb) is applied to the current source 19, electrical current Ibis supplied to the laser diode LD which emits the laser beam having theintensity Pb (see FIG. 4).

The image data supplied to the intensity modulating D/A converter 44 istransmitted from the printer controller PC through a three-state buffer45. The three-state buffer 45 functions as a switch, and when a statusof an input control signal OE is logical-high, the three-state buffer 45transmits the image data D0 through D7 to the intensity modulating D/Aconverter 44 and to the digital comparator 46 as is.

If the status of the input control signal OE is logical-low, thethree-state buffer 45 is in an OFF condition (high-impedance condition),and a pull-up voltage (logical-high level) at the output side of thethree-state buffer 45 is applied to the D/A converter 44 and to thedigital comparator 46. In other words, all of the image data D0 throughD7, which are input to the D/A converter 44 and the digital comparator46, are set to logical-high when the input control signal OE islogical-low.

To the intensity modulating D/A converter 44, a blanking signal BLK istransmitted from the CPU 150 as a control signal. If a status of theblanking signal BLK is logical-high, the intensity modulating D/Aconverter 44 outputs values in accordance with the image data D0 throughD7, which is transmitted from the three-state buffer 45, and the outputvoltage vb of the black level setting D/A converter 43. When theblanking signal BLK is logical-low, the output voltage of the intensitymodulating D/A converter 44 becomes zero (0).

A digital comparator 46 works synchronously with the data transfer clockCLK. The digital comparator 46 turns OFF the switch element 18 only whenevery bit of the image data D0 through D7 has a value zero.

To the positive terminals of the comparators 14 and 15, the voltageproportional to the electrical current available through the photo diodePD is applied, respectively. To the negative terminals of thecomparators 14 and 15, the output voltages V1 and V2 of the D/Aconverters 16 and 17 are applied, respectively. The first voltage V1 issmaller than the second voltage V2. The comparators 14 and 15 comparethe voltage proportional to the electrical current flowing through thephoto diode PD with the first and the second voltages, respectively, andtransmit the comparison result into the CPU 150. Namely, whether thevoltage proportional to the electrical current available through thephoto diode PD is lower than voltage V1, between voltages V1 and V2, orgreater than voltage V2 is determined based on the output values of thecomparators 14 and 15.

The quantities of voltages V1 and V2 are determined in accordance withdata transmitted from the CPU 150 to the D/A converters 16 and 17,respectively. If voltage V1 is set to a voltage corresponding to theintensity Pw of the laser beam, and voltage V2 is set to a voltagecorresponding to the intensity Pb of the laser beam, the intensity ofthe laser diode LD becomes Pw and Pb can be detected based on the outputof the comparators 14 and 15 when the APC is executed.

In the embodiment, the reference voltages applied to the negativeterminals of the comparators 14 and 15 are the output voltages of theD/A converters 16 and 17, respectively. However, it is not limited tothis configuration. If the voltages are known in advance, circuits forgenerating such voltages can be employed instead of the D/A converters16 and 17.

In the embodiment, the reference voltages are made changeable. Thisconfiguration has an advantage such that the density of the entire imagecan be changed by changing the reference voltages. Further, thesensitivity of the photoconductive material can also be compensated forby changing the reference voltages.

The APC operation will be described. Note that a value of the variableresistor 21 has been adjusted, and the output voltages of the D/Aconverters 16 and 17 have also been determined.

If the scanning speed is relatively slow, the adjustment of the whitelevel and the black level can be done within one scanning operation,after an image forming, and before the next horizontal synchronoussignal HSYNC.

First, a laser beam is received by the light receiving sensor 7 of thewrite position detecting circuit 13 and the horizontal synchronoussignal HSYNC is generated. Then, according to the horizontal synchronoussignal HSYNC and synchronously with the transfer clock CLK, the imagedata D0 through D7 is sequentially transferred to the laser scanningunit SU from the image memory 31 of the printer controller PC. After theimage forming operation in accordance with transferred image data hasbeen finished, the APC is effected.

Assume that the APC for the white level intensity Pw is performed first.

In order to execute the APC, the control signal OE is set to logical-lowto set the three-state buffer to the high impedance. With this setting,regardless of the input image data D0 through D7, the three-state buffer45 are ready to output data D0 through D7 each have value of H(logical-high). Then, the CPU 150 turns ON the switch element 18. Thelaser diode LD then emits the laser beam continuously. At this time, theblanking signal BLK input to the intensity modulating D/A converter 44is set to logical-low state to make the output voltage of the intensitymodulating D/A converter 44 to be zero. With this control, data 0 (zero)is input to the digital comparator 46.

Assume that some data is input to the white level setting D/A converter41. The output voltages of the D/A converter 41 and the intensitymodulating D/A converter 44 are input to the adder 42. However, sincethe output voltage of the intensity modulating D/A converter 44 is zero,the output voltage of the adder 42 is equal to the output voltage of theD/A converter 41, i.e., Vw.

Since the reference voltage of the comparator 14 corresponds to thewhite level intensity Pw of the laser beam, by increasing and decreasingthe input data of the D/A converter 41, the point where the output ofthe comparator 14 switches from logical-high to logical-low, or fromlogical-low to logical-high may be changed. The point where the outputof the comparator 14 switches is the point where the laser diode LDemits the laser beam having the white level intensity Pw.

If the output of the comparator 14 when the APC is started islogical-low, the value of the input data of the D/A converter 41 isincreased so that the intensity of the laser beam gradually increases.The input data to the D/A converter 41 when the output of the comparator14 switches from logical-low to logical-high is considered to be thedata for the white level intensity setting data.

If the output of the comparator 14 when the APC is started islogical-high, the value of the input data of the D/A converter 41 isdecreased so that the intensity of the laser beam gradually decreases.In this case, the input data to the D/A converter 41 when the output ofthe comparator 14 switches from is logical-high to logical-lowconsidered to be the data for white level intensity setting data.

When the white level setting data is determined as above, the value isfixed and then the APC for the black level intensity will be performed.

The APC for determining the black level intensity setting data isexecuted while maintaining the output data Vw of the D/A converter 41 asthe value obtained in the APC for the white level intensity. Firstly,the control signal OE to be transmitted to the three state buffer 45 isset to logical-low to set the three-state buffer 45 into the highimpedance state. With this setting, all the bit data to be input to theD/A converter 44 and to the digital comparator 46 becomes 1 (i.e., thedecimal value of the input data becomes 255).

As described above, the intensity modulating D/A converter 44 outputsthe multiplication of the output voltage of the D/A converter 43 and theinput data value/255. Since the input data value is 255, the intensitymodulating D/a converter 44 outputs the voltage of the D/A converter 43.The adder 42 therefore outputs Vb+Vw. In the APC for the black levelintensity, the voltage Vb is adjusted such that the intensity of thelaser beam coincides with the value Pb.

The reference voltage input to the comparator 15 (i.e., the outputvoltage of the D/A converter 17) corresponds to the voltage across thevariable resistor 21 when the laser diode emits the laser beam havingthe intensity of Pb. Therefore, if a point where the output of thecomparator 15 switches from logical-high to logical-low, or logical-lowto logical-high is detected, the point can be considered to be the pointwhere the laser diode Ld output the laser beam having the black levelintensity Pb. Similar to the APC for the white level intensity, byincreasing/decreasing the input data of the D/A converter 43, theintensity of the laser beam can be varied. If the input data of the D/Aconverter 43 which make the intensity of the laser beam emitted by thelaser diode LD to be Pb is detected, the APC for the black levelintensity Pb is finished. The data to be input to the D/A converter 43thus detected is fixed.

The image forming operation will be described.

After the APC is completed, the laser diode LD is controlled to emitmodulated laser beam in accordance with the voltages Vw and Vb, and theimage data D0 through D7.

The CPU 150 sets the control signal OE input to the three-state buffer45 to be logical-high in order to allow the data transmission from theimage memory 31 to the three-state buffer 31. When the intensitymodulating D/A converter 44 receives the image data D0 through D7 by wayof the three-state buffer 45, voltage Vd expressed as follows is output.

    Vd=Vb×DD/255

where, DD is a decimal value represented by the image data D0 throughD7.

The adder 42 adds the output voltage vd of the intensity modulating D/Aconverter 44 and output voltage Vw of the D/A converter 41, and outputsthe voltage Vadd.

    Vadd=Vw+(Vb×DD/255)

The output voltage Vadd of the adder 42 is input to the current source19. Therefore, according to the value of the input data D0 through D7,the intensity of the laser beam varies within a range defined between aminimum value Pw and a maximum value Pb. However, according to the abovedescribed control, when the value represented by the data D0 through D7is zero, the laser beam has the intensity of Pw. This situation mightnot accurately produce a white image because Pw is not zero. To avoidthis problem, when all the bits of the input image data D0 through D7are zero, a control signal is sent from the digital comparator 46 to theswitching element 18 to turn OFF the switching element 18. Thus, whenthe value represented by the image data D0 through D7 is zero, the laserdiode LD is controlled not to emit the laser beam, and a perfect whitelevel is achieved.

The image forming operation and the APC operation is executed asdescribed above, if the image data has a value greater than zero, andthe electrical current to be passed through the laser diode LD varieswithin the range between Iw and Ib which are greater than the thresholdvalue Ith. Thus, intensity modulating using an electrical current lowerthan the threshold value Ith is avoid. Therefore, the gradation imagecan be produced accurately. Further, if image data represents a whiteimage, the laser diode LD is controlled not to emit the laser beam.Therefore, the white image can also be produced accurately without a fogeffect. Furthermore, even if the sensitivity of the photoconductive drumis changed due to a replacement thereof or due to the deterioration withage, by varying the input data of the D/A converters 16 and 17, thechange of the sensitivity of the photoconductive drum can easily becompensated.

Since the APC operation is executed at each scanning, the change of thecharacteristic of the laser diode LD due to the change of theoperational environment does not affect the density of the image.

FIG. 6 shows the change of the I-C characteristic of the laser diode LDdue to the change of the ambient temperature. In FIG. 6, acharacteristic CH1 (indicated by a solid line) is a characteristic whenthe ambient temperature is relatively low, and a characteristic CH2(indicated by a broken line) is a characteristic when the ambienttemperature is relatively high. As described before, the characteristicCH2 is obtained by shifting the characteristic in a right-hand directionin FIG. 6 by the amount of ΔI. As shown in FIG. 6, since the graph forthe characteristic CH2 is shifted rightward with respect to thecharacteristic CH1, the threshold current value Ith for thecharacteristic CH1 is also increased, in the characteristic CH2, to avalue Ith'. The inclination (which is called as a differentialefficiency) of the graphs are considered to be substantially the same.

According to the characteristic CH1, when the electrical current Im isavailable in the laser diode LD, the intensity of the laser beam is theminimum intensity (white level intensity) Pw, which is indicated as apoint A. When the electrical current Im+Ia is available in the laserdiode LD, the intensity of the laser beam is the maximum intensity (theblack level intensity) Pb which is indicated as a point B.

In order to have the similar intensity of the laser beam in accordancewith the characteristic CH2, the electrical current corresponding to thepoints A' to B' on the graph should be made flow through the laser diodeLD.

The electrical current corresponding to point A' and B' are Im' andIm'+Ia, which are expressed as follows.

    Im'=Im +ΔI

    Im'+Ia=Im+ΔI +Ia

As described above, when the ambient temperature changes, ΔI is to betaken into account. According to the embodiment, a sum of the currentsIm+ΔI corresponds to the voltage Vw, and the electrical current Iacorresponds to the voltage Vb. The current value available in the laserdiode LD corresponds to the output voltage Vadd of the adder 42 which isexpressed as follows.

    Vadd =Vw+(Vb×DD/255)

Therefore, the change of the current value due to the change of theambient temperature is compensated only by changing the voltage Vw whichcorresponds to the white level intensity.

FIG. 7 is a flowchart illustrating a first embodiment of the APCoperation to be executed in the laser scanning unit SU. As shown in FIG.2, within one scanning period (i.e., between two successive horizontalsynchronous signals), the image forming operation is performed first,and then within the remaining period, the APC operation is performed.

At S23, a predetermined value is set as a data value to be input to theD/A converter 41. At the initial stage, the value of the input data tothe D/A converter 41 is set to a default value, and the data used in theprevious APC operation is used as the input data to the D/A converter 41in the succeeding APC operations. The input data to be input to the D/Aconverter 43 has a value which is a sum of the value of the input datato the D/A converter 41+a predetermined voltage value. The predeterminedvoltage value corresponds to the difference of the current valuesbetween the points A and B, or A' and B' of FIG. 6. In the firstembodiment, with maintaining the above relationship, the input data tothe D/A converter 41 is changed, and the output of the comparator 15 ismonitored (S25). Specifically, the control signal OE and the blankingsignal BLK are set to logical-low. Then, depending on the output valueof the comparator 15, the data value input to the D/A converter 41 isincreased or decreased (S25). For example, if the output of thecomparator 15 is L when the APC is started, the data value input to theD/A converter 41 is increased, and detects when the output data of thecomparator 15 changes from logical-low to logical-high.

When the output value of the comparator 15 switches between logical-lowand logical-high, the data input to the D/A converter 41 is determinedto be the white level setting data.

In the first embodiment, the voltage value vb of the D/A converter 43 isfixed and only the voltage value Vw output from the D/A converter 41 isadjusted such that the black level intensity Pb is obtained. Therefore,using only one APC operation, both the white level intensity and theblack level intensity can be adjusted simultaneously.

FIG. 8 shows a flowchart illustrating the APC operation according to asecond embodiment of the present invention.

In the second embodiment, the scanning speed is relatively fast, andonly one of the white level intensity or the black level intensity isadjusted within one scanning operation. In other words, the APCoperations for black level intensity and the white level intensity areexecuted alternately.

At step S31, the APC operation for a level intensity executed based onthe APC operation of which of the black or the white level intensitieswas previously executed. When the previous APC operation is for theblack level intensity, the APC operation for the white level operationis executed (S31:YES). In such a case, the blanking signal BLK is set tological-low and the APC operation for the white level intensity isexecuted at S33 through S37.

Similarly to the first embodiment, as shown in FIG. 2, within onescanning period (i.e., between two successive horizontal synchronoussignals), the image forming operation is performed first, and thenwithin the remaining period, the APC operation is performed.

In order to execute the APC operation for the white level intensity, thecontrol signal OE is set to logical-low, the switch 18 is closed, andthe blanking signal BLK is set to logical-low. At S33, at the initialstage, the value of the input data to the D/A converter 41 is set to adefault value, and the data used in the previous APC operation for whitelevel intensity is used as the input data to the D/A converter 41 in thesucceeding APC operations for white level intensity. At S35, the inputdata to the D/A converter 41 is increased or decreased. Specifically, ifthe comparator 14 outputs logical-high level signal, the input data isdecreased, and if the comparator 14 outputs logical-low level signal,the input data is increased. Then, at S37, the CPU 150 determineswhether the voltage across the variable resistor 21 reaches voltage V1by detecting when the output signal of the comparator 14 has changedfrom logical-high to logical-low, or logical-low to logical-high. If theoutput signal of the comparator 14 has not been changed, the CPU 150determines that the voltage across the variable resistor 21 does notreach the voltage V1, i.e., the intensity of the laser beam emitted bythe laser diode LD does not have the white level intensity (S37:NO).Then step S35 is executed again and the voltage applied to the currentsource 19 is changed, and the intensity is examined again at S37.

If the intensity of the laser beam reaches the white level, step S37determines YES, and then the APC operation is completed.

If the APC operation for the black level intensity is to be executed(S31:NO), then the control signal is set to logical-low, and theblanking signal BLK is set to logical-high and the APC operation for theblack level intensity is executed at S41 through S45. At S41, in orderto execute the APC operation for the black level intensity, the value ofthe input data to the D/A converter 43 is set to a default value whenthe APC for the black level intensity is executed, and the data used inthe previous APC operation for the black level intensity is used as theinput data to the D/A converter 43 in the succeeding APC operations forthe black level intensity. At S43, the input data to the D/A converter43 is increased or decreased. Specifically, if the comparator 15 outputslogical-high level signal, the input data is decreased, and if thecomparator 15 outputs logical-low level signal, the input data isincreased. Then, at S45, the CPU 150 determines whether the voltageacross the variable resistor 21 reaches voltage V2 by detecting when theoutput signal of the comparator 15 has changed from logical-high tological-low, or logical-low to logical-high. If the output signal of thecomparator 15 has not been changed, the CPU 150 determines that thevoltage across the variable resistor 21 does not reach the voltage V2,i.e., the intensity of the laser beam emitted by the laser diode LD doesnot have the black level intensity Pb (S21:NO). Then steps S43 and S45are executed until the intensity of the laser beam emitted by the laserdiode LD reaches the black level intensity Pb.

FIGS. 9A through 9F are timing charts showing data communication betweenthe printer controller PC and the laser scanning unit SU through thecommunication line COM.

When the laser beam printer 100 is turned ON (FIG. 9A), the main CPU 30of the printer controller PC transmits initiation data R1 to the CPU 150of the laser scanning unit SU (FIG. 9B). The CPU 150 executes anoperation C1 (see FIG. 9F) which is a data receiving operation when theinitiation data is transmitted from the CPU 30. In the operation C1, thepolygonal mirror 3 is started to rotate, and laser diode is turned ON.When the laser beam printer 100 is ready to execute a printing job (whenthe rotation of the polygonal mirror is stabilized, and the laser beamis stably emitted by the laser diode), the CPU 150 transmits a readysignal T1 (see FIG. 9C) after the data transmitting operation C2 (FIG.9F).

The main CPU 30 transmits an operation checking data RC (see FIG. 9B) tothe laser scanning unit SU during printing jobs. The CPU 150 of thelaser scanning unit SU switches its operation from the main operation Mto the data receiving operation C1 when the operation checking data RCis transmitted (FIGS. 9B and 9F), and examines the current operation. Ifthe current operation is normally executed, the CPU 150 transmits thedata TC indicating that the operation is normally executed in the datatransmitting operation C2 (FIGS. 9C and 9F), and if an operation errorhas been occurred, the CPU 150 transmits an NG data in the datatransmitting operation C2.

The operation checking data RC includes data related to the change ofthe sensitivity of the photoconductive drum 4 as well as other datanecessary for examining the operation. The CPU 150 changes the inputdata to the D/A converters 16 and 17 in accordance with the data relatedto the change of the sensitivity of the photoconductive drum.Accordingly, the reference voltages applied to the comparators 14 and 15are changed to meet the sensitivity of the photoconductive drum 4. Thedata related to the change of the sensitivity of the photoconductivedrum 4 is included in the operation checking data RC only when thechange of the input data to the D/A converters 16 and 17 becomesnecessary. In the NG data, data related to the part of the laserscanning unit SU which does not operate correctly is also included.

The data communication between the printer controller PC and the laserscanning unit SU is mainly executed during the printing operation whenthe load on the CPU 150 is relatively small. Areas indicated as CM inFIG. 9D are periods where the data communication can be done, and areasPR are periods where the printing is executed.

FIG. 9D shows a laser ON/OFF signal. The signal is an active-low signal,and when /VIDEO is "L", the laser diode LD is turned ON. The APCoperation is executed during the periods AP (see FIGS. 9D, and 10F). Inthis timing chart, the scanning speed is relatively high, and thereforethe period AP is relatively short. Accordingly, the APC operation asillustrated in FIG. 8 is employed, and the data values to be supplied tothe D/A converters 41 and 43 are alternately determined.

During each period AP, the APC operation has a higher priority than thedata communication operation. That is, while the APC operation isexecuted, the data communication operation cannot be executed. Further,if the data communication operation and the APC operation are to startsimultaneously, the APC operation is executed, and after the completionof the APC operation, the data communication is executed.

In the embodiments described above, in response to the operationchecking signal R1 transmitted by the main CPU 30, the CPU 150 transmitsdata related to the operating condition of the laser scanning unit SU tothe main CPU 30. Therefore, for example, the laser diode LD is notturned ON, if the laser diode LD is deteriorated or because the /VIDEOsignal is "H" is transmitted to the main CPU 30.

According to the embodiments described above, the white level intensitycorresponding to the minimum intensity of the laser beam and the blacklevel intensity corresponding to the maximum intensity of the laser beamare set, and the range defined by the white level intensity and theblack level intensity is divided by the number of gradation steps, anaccurate gradation image can be obtained regardless of the I-Ccharacteristic of the laser diode, and regardless of the fluctuation ofthe characteristic due to the change of the ambient temperature.

Further, according to the second embodiment, even if the scanning speedis relatively high, the black and white level intensities can beadjusted accurately without slowing down the scanning speed.

Furthermore, if the image data represents a white pixel, the laser diodeis controlled not to emit the laser beam. Accordingly, a so-called fogphenomena on the latent image will not occur, and the qualified imagecan be obtained.

Still further, since the intensity of the laser beam can be compensatedin accordance with the change of the sensitivity of the photoconductivedrum, the produced image has the desired gradation.

The present disclosure relates to subject matter contained in JapanesePatent Application No. HEI 7-246693, filed on Aug. 31, 1995, which isexpressly incorporated herein by reference in its entirety.

What is claimed is:
 1. A laser scanning unit for repeatedly scanning apredetermined area of a surface, comprising:a laser diode that emits alaser beam; a laser diode driver that drives said laser diode to emitsaid laser beam with an intensity related to a gradation step of aplurality of predetermined gradation steps of image data representing animage, said plurality of gradation steps defining an intensity range ofsaid laser beam defined by a first intensity and a second intensitygreater than said first intensity; a first adjusting system that adjustssaid first intensity of said laser beam to a first predeterminedintensity; a second adjusting system that adjusts said second intensityof said laser beam to a second predetermined intensity; and an adjustingsystem controller that actuates only one of said first adjusting systemand said second adjusting system during each scanning operation.
 2. Thelaser scanning unit according to claim 1, wherein said first and secondadjusting systems are actuated when said laser beam does not scan saidpredetermined scanning area of said surface to be scanned.
 3. The laserscanning unit according to claim 1, wherein if said image datarepresents a zero value, an intensity of said laser beam is less thansaid first predetermined intensity.
 4. The laser scanning unit accordingto claim 3, wherein if said image data represents said zero value, saidlaser diode controller controls said laser diode not to emit said laserbeam.
 5. The laser-scanning unit according to claim 1, wherein saidfirst and second intensities are variable.
 6. The laser scanning unitaccording to claim 5, wherein said surface to be scanned comprises aphotoconductive surface, said first intensity of said laser beam andsaid second intensity of said laser beam being adjusted based upon asensitivity of said photoconductive surface.
 7. The laser scanning unitaccording to claim 1, wherein said first intensity is greater than anintensity of a laser beam which is emitted when a threshold electricalcurrent is available in said laser diode.
 8. The laser scanning unitaccording to claim 1, wherein said first adjusting system comprises:adetector for detecting the intensity of said laser beam; a comparatorfor comparing said intensity of said laser beam with said firstpredetermined intensity; and a controller for controlling said laserdiode driver to change said intensity of said laser beam in accordancewith a comparison result of said comparator.
 9. The laser scanning unitaccording to claim 1, wherein said second adjusting system comprises:adetector for detecting the intensity of said laser beam; a comparatorfor comparing said intensity of said laser beam with said secondpredetermined intensity; and a controller for controlling said laserdiode driver to change said intensity of said laser beam in accordancewith a comparison result of said comparator.
 10. The laser scanning unitof claim 1, wherein said a first adjusting system and said secondadjusting system are alternately actuated for successive scanningoperations.
 11. A laser scanning unit for repeatedly scanning apredetermined area of a surface, comprising:a laser diode that emits alaser beam; a laser diode driver that drives said laser diode to emitsaid laser beam with an intensity related to a gradation step of aplurality of predetermined gradation steps of image data representing animage, said plurality of gradation steps defining an intensity range ofsaid laser beam defined by a first intensity and a second intensitygreater than said first intensity a first adjusting system that adjustssaid first intensity of said laser beam to a first predeterminedintensity; a second adjusting system that adjusts said second intensityof said laser beam to a second predetermined intensity; and an adjustingsystem controller that actuates one of said first adjusting system andsaid second adjusting system during each scanning operation of aplurality of scanning operations, wherein said first adjusting systemand said second adjusting system are alternately actuated for successivescanning operations.
 12. The laser scanning unit of claim 11, whereinsaid surface being scanned comprises a surface of a photoconductivedrum.
 13. The laser scanning unit of claim 12, wherein said firstintensity of said laser beam and said second intensity of said laserbeam are adjusted in accordance with a sensitivity of said surface ofsaid photoconductive drum.
 14. The laser scanning unit of claim 11,wherein said first adjusting system comprises:a detector that detectssaid intensity of said laser beam; a comparator that compares saidintensity of said laser beam with said first predetermined intensity;and a controller that controls said laser diode driver to change saidintensity of said laser beam in accordance with a comparison result ofsaid comparator.
 15. The laser scanning unit of claim 11, wherein saidsecond adjusting system comprises:a detector that detects said intensityof said laser beam; a comparator that compares said intensity of saidlaser beam with said second predetermined intensity; and a controllerthat controls said laser diode driver to change said intensity of saidlaser beam in accordance with a comparison result of said comparator.16. A laser scanning unit for repeatedly scanning a predetermined areaof a surface, comprising:a laser diode that emits a laser beam; a laserdiode driver that drives said laser diode to emit said laser beam withan intensity related to a gradation step of a plurality of predeterminedgradation steps of image data representing an image, said plurality ofgradation steps defining an intensity range of said laser beam definedby a first intensity and a second intensity greater than said firstintensity; a first adjusting system that adjusts said first intensity ofsaid leaser beam to a first predetermined intensity, a second adjustingsystem that adjusts said second intensity of said laser beam to begreater than said first predetermined intensity by a predeterminedamount; and an adjusting system controller that actuates only one ofsaid first adjusting system and said second adjusting system during eachscanning operation of a plurality of scanning operations.
 17. The laserscanning unit according to claim 16, wherein if said image datarepresents a zero value, an intensity of said laser beam is less thansaid first predetermined intensity.
 18. The laser scanning unitaccording to claim 17, wherein if said image data represents said zerovalue, said laser diode controller controls said laser diode not to emitsaid laser beam.
 19. The laser scanning unit according to claim 16,wherein said first and second intensities are variable.
 20. The laserscanning unit according to claim 19, wherein said surface to be scannedcomprises a photoconductive surface, said first intensity of said laserbeam and said second intensity of said laser beam being adjusted basedupon a sensitivity of photoconductive surface.
 21. The laser scanningunit according to claim 16, wherein said first intensity is greater thanan intensity of a laser beam which is emitted when a thresholdelectrical current is available in said laser diode.
 22. The laserscanning unit of claim 16, wherein said first adjusting system and saidsecond adjusting system are alternately actuated for successive scanningoperations of said plurality of scanning operations.